156 FOETAL MEMBRANES. 



the still wide splanchnic stalk. At the end of the stalk, which is as yet short, it bends 

 outwards again and spreads over the surface of the yolk. The somatopleure, folded in less 

 than the splanchnopleure to form the wider somatic stalk, sooner bends round and runs 

 outwards again. At a little distance from both the head and the tail it is raised up 

 into a fold, af, af, that in front of the head being the highest. These are the arnniotic 

 folds. Descending from either fold, it speedily joins the splanchnopleure again, and 

 the two, once more united into an uncleft membrane, extend some way downwards over 

 the yolk, the limit or outer margin of the opaque area not being shewn. All the space 

 between the somatopleure and the splanchnopleure is shaded with dots, pp. Close to 

 the body this space may be called the pleuroperitoneal cavity; but outside the body it 

 runs up into either anmiotic fold, and also extends some little way over the yolk. 



D represents the tail-end at about the same stage on a more enlarged scale, in order 

 to illustrate the position of the allantois nl (which was for the sake of simplicity omitted 

 in C), shewn as a bud from the splanchnopleure, stretching downwards into the pleuro- 

 peritoneal cavity pp. The dotted area representing as before the whole space between 

 the splanchnopleure and the somatopleure, it is evident that a way is open for the 

 iillantois to extend from its present position into the space between the two limbs of the 

 amniotic fold af. 



E, also a longitudinal section, represents a stage still farther advanced. Both 

 splanchnic and somatic stalks are much narrowed, especially the former, the cavity of 

 the alimentary canal being now connected with the cavity of the yolk by a mere canal. 

 The folds of the amnion are spreading over the top of the embryo and nearly meet. 

 Each fold consists of two walls or limbs, the space between which (dotted) is as before 

 merely a part of the space between the somatopleure and splanchnopleure. Between 

 these arched arnniotic folds and the body of the embryo is a space not as yet entirely 

 closed in. 



F represents on a different scale a transverse section of E taken through the middle 

 of the splanchnic stalk. The dark ring in the body of the embryo shews the position 

 of the neural canal, below which is a black spot, marking the notochord. On either 

 side of the notochord the divergence of somatopleure and splanchnopleure is obvious. 

 The splanchuopleure, more or less thickened, is somewhat bent in towards the middle 

 line, but the two sides do not unite, the alimentary canal being as yet open below at 

 this spot; after converging somewhat they diverge again and run outwards over the 

 yolk. The somatopleure, folded in to some extent to form the body-walls, soon bends 

 outwards again, and is almost immediately raised up into the lateral folds of the 

 arnnion af. The continuity of the pleuroperitoneal cavity, within the body, with the 

 interior of the amniotic fold, outside the body, is evident ; both cavities are dotted. 



G, which corresponds to D at a later stage, is introduced to shew the manner in 

 which the allantois, now a considerable hollow body, whose cavity is continuous with 

 that of the alimentary canal, becomes directed towards the amniotic fold. 



In H a longitudinal, and I a transverse section of later date, great changes have 

 taken place. The several folds of the amnion have met and coalesced above the body 

 of the embryo. The inner limbs of the several folds have united into a single 

 membrane (a), which encloses a space (ae) round the embryo. This membrane a is the 

 amnion proper, and the cavity within it, i.e. between it and the embryo, is the cavity 

 of the amnion containing the liquor ainnii. The allantois is omitted for the sake of 

 simplicity. 



It will be seen that the aniniou a now forms in every direction the termination of 

 the somatopleure; the peripheral portions of the somatopleure, the imited outer or 

 descending limbs of the folds af in C, D, F, G having been cut adrift, and now forming 

 an independent continuous membrane, the serous membrane, immediately underneath 

 the vitelline membrane. 



In I the splanchuopleure is seen converging to complete the closure of the ali- 

 mentary canal a' even at the stalk (elsewhere the canal has of course long been closed 

 in), and then spreading outwards as before over the yolk. The point at which it unites 

 with the somatopleure, marking the extreme limit of the cleavage of the mesoblast, is 

 now much nearer the lower pole of the diminished yolk. 



As a result of these several changes, a great increase in the dotted space has taken 

 place. It is now possible to pass from the actual peritoneal cavity within the body, on 

 the one hand round a great portion of the circumference of the yolk, and on the other 

 hand above the amnion a, in the space between it and the serous envelope. 



Into this space the allantois is seen spreading in K at al. 



In L the splanchnopleure has completely invested the yolk- sack, but at the lower 

 pole of the yolk is still continuous with that peripheral remnant of the somatopleure 



